The present invention relates to an improved method of producing thermoplastic particles, particularly suitable for use as a writing nib, and to a process for the production of a material from such particles.
Porous plastic structures, including writing nibs, are commonly manufactured by sintering together granular particles of a thermoplastic material. "Kynar", a trade name of Pennwalt Corporation, is commonly used as a thermoplastic material of such structures. One prior art process involves chopping and grinding the "Kynar" material by means of a pulverizer to produce granular particles. It is believed that the maximum density can be approached only where such granular particles are spherical in shape. However, the granular material of the prior art process is composed of particles that are irregularly shaped or porous and therefore maximum density within a mold is not possible.
In the sintering step, the particles tend to change toward a shape having a minimum surface area (spherical) which inherent tendency causes the entire mass to shrink. This phenomenon results in irregularly shaped products. Another problem associated with this prior art process stems from the fact that the individual particles themselves are porous. The sintering step also tends to reduce the porosity of the particles, further contributing to the shrinkage problem.
Yet another problem associated with such prior art processes is the inability to control the pore sizes of the finished product due to its construction from irregularly shaped particles. It is desirable to control pore size in order to provide a controlled flow when the product is used for gas or liquid flow control or fluid transfer. If the pourous material is to be used as a filter, it is also desirable to control the pore size.